Why do I have HEIC and JPG versions of every photo?

You can select them first with a smart album to make them easier to delete, but before we do that we should try to understand why it has happened, or it may just happen again.

This can also happen if you are mixing transfer methods - for example iCloud and USB. or USB and Air drop.

The fact your app is so slow on a very fast machine may indicate that the library is damaged, or is stored in an incompatible location.

How are you transferring photos from your phone?

Are all photos duplicated, or just some?

Where is your library stored? In your pictures folder, or somewhere else. If on an external drive, what is the format?

Have you used any speed up or clean up apps, or duplicate removal apps?

Are you using iCloud Photos or My PhotonStream and are manually importing the photos from the iPhone as well? Do the HEIC file and the JPEG file have the same pixel size (width and height), or is the jpeg smaller?

This normally happens when your phone is taking HEIC format pictures, but converting them on download to your mac to JPG Then when you connect your phone again, they are different filenames, so not recognised as already downloaded.

To fix this you need to change the iphone photos transfer setting to "keep original" from "automatic"

Then delete the jpg versions of the photos from your mac.